As the old complaint that men's long hairstyles make it impossible to tell ""if it's a boy or a girl"" reveals, gender ambiguity is socially unsettling to many people. Boldly stepping into the breach, Fausto-Sterling contends that the fear of gender confusion has pushed science and medicine to go to extreme lengths in constructing solid concepts of sex (i.e., an individual's anatomical attributes) and gender (i.e., the internal conviction of one's maleness or femaleness). As in her now classic book, Myths of Gender, Fausto-Sterling draws on a wealth of scientific and medical information, along with social, anthropological and feminist theory, to make the case that ""choosing which criteria to use in determining sex, and choosing to make the determination at all, are social decisions for which scientists can offer no absolute guidelines."" Further, she adds, ""our beliefs about gender affect what kind of knowledge scientists produce about sex in the first place."" While the book encompasses a wide range of topics--including a cultural history of hermaphroditism (now more properly termed ""intersexuality"") and the recent medical interventions used to ""cure"" it, an account of the emergence of sex hormone research and its use to create changes in sexual orientation, and the history of how science has (mis)understood the brain in terms of gender--Fausto-Sterling's cogent use of concrete historical examples, her simple language and personal anecdotes keep this complex synthesis accessible. Her insightful work offers profound challenges toscientific research, the creation of social policy and the future of feminist and gender theory. (Feb.)